Thick film conductors are widely used as a means of interconnecting various passive and active devices for hybrid microcircuits and resistor networks. Utility as a general purpose conductor requires certain performance attributes such as conductivity, solderability, solder leach resistance, compatibility with other circuit components and ability to be processed under a wide range of conditions. Inherent in the usefulness of thick film conductors is the cost of materials in the composition. It is extremely advantageous to reduce the cost without significantly changing the performance characteristics.
Thick film conductors are comprised of conductive metal and inorganic binder, both of which are in finely divided form and are dispersed in an organic medium. The conductive metal is ordinarily gold, palladium, silver, platinum or mixtures and alloys thereof, the choice of which depends upon the particular combination of performance characteristics which are sought, e.g., resistivity, solderability, solder leach resistance, migration resistance, adhesion and the like.
Thick film techniques are contrasted with thin film techniques which involve deposition of particles by vacuum evaporation or sputtering. Thick film techniques are discussed in Handbook of Materials and Processes for Electronics, C. A. Harper, Editor, Mc-Graw-Hill, N.Y. 1970, Chapter 12.
In the current economic climate in which noble metals have experienced substantial fluctuations in price, it is especially attractive from a business viewpoint to substitute less expensive base metals as the conductive metal in thick film conductor compositions. Several base metals have been proposed and used with mixed success as the conductive phase for thick film conductors. Among these the most important is copper which has been formulated in a number of different ways for a wide variety of applications. For example, U.S. Pat. No. 2,993,815 to Treptow is directed to a method of forming a copper conductive layer for printed circuits on a refractory substrate by screen printing a layer of 5-50 parts by weight copper or copper oxide and 1 part by weight of reduction-resistant glass frit dispersed in an organic medium. The conductive layer is formed by firing the applied paste in two stages at 500.degree.-1050.degree. C. On the other hand, U.S. Pat. No. 3,647,532 to Friedman is directed to conductive inks for use on ceramic substrates comprising copper and glass frit dispersed in an organic polymeric binder using a lead borosilicate glass binder containing cadmium oxide. Firing is carried out in a nonoxidizing atmosphere at 820.degree.-850.degree. C.
Bolon et al., U.S. Pat. No. 3,988,647, disclose a conductor composition comprising copper particles which have been treated to remove oxide from the surface dispersed in a solventless polymeric binder. U.S. Pat. No. 4,070,518 to Hoffman is directed to a conductor composition especially for use on dielectric substrates, comprising 85-97% wt. Cu powder and 3-15% wt. of cadmium- and bismuth-free lead aluminoborate glass frit dispersed in an organic medium.
In Grier, U.S. Pat. No. 4,072,771, a conductor composition is disclosed comprising copper particles which have been preoxidized to form a surface layer of CuO and glass frit dispersed in 15-25% wt. organic medium. U.S. Pat. No. 4,172,919 to Mitchell is directed to a conductor composition comprising 86-97% wt. Cu powder, 1-7% wt. CuO and 1-7% wt. of a glass frit containing at least 75% Bi.sub.2 O.sub.3 dispersed in 10-30% wt. inert organic medium.
In EPO application No. 0068167 to McCormick et al., a conductor composition is disclosed comprising 65-80 pbw Cu powder, 0-6 pbw CuO and 3-8 pbw of a Bi-free, low melting glass dispersed in an organic medium containing 20-40% wt. methacrylate resin dissolved in volatile solvent.
In applicant's copending U.S. patent application Ser. No. 505,730 filed June 20, 1983, there is disclosed a thick film conductor composition comprising finely divided particles of copper oxide-coated copper and low softening point inorganic binder dispersed in a low resin organic medium.
When copper conductors such as those described above are used in microcircuits, they are frequently subjected to quite rigorous conditions during fabrication. For example, in a typical application, the copper-containing composition is printed on a substrate, dried and fired in a nitrogen atmosphere at 900.degree. C. Then a pattern of resistor material is printed in proper registry atop the conductor layer and the copper-containing composition and overlying layer of resistor material are fired at about the same temperature in nitrogen to effect sintering of the resistor material. Following this, an overglaze may be applied and the entire assemblage is fired in nitrogen once again to sinter the overglaze material. When this is completed, leads are soldered onto the conductive layer. Thus, in this typical situation, the copper is subjected to as many as three high temperature firings and, in some fabrications, the copper-containing layer may be subjected to as many as five such firings. Because of minute amounts of oxygen which remain in the nitrogen firing atmospheres, the copper conductive surface becomes progressively more oxidized. In turn, the progressive oxidation of the copper components of the conductor composition causes poor solderability. In the past, it was necessary to fire in a very pure nonoxidizing atmosphere in a short firing cycle to lessen such oxidation or to limit the use of the composition to applications which do not require multiple firing.